1. Field of the Invention
The invention relates to a spray powder on a tungsten carbide basis and to a method for the manufacture of such a spray powder for the thermal coating of a substrate, in particular for the thermal coating of a brake disc of a vehicle, as well as to a substrate, in particular a brake disc having a thermal spray layer on a tungsten carbide basis, and to a method for the manufacture of a thermal spray layer in accordance with the preamble of the independent claim of the respective category.
2. Discussion of Background Information
Coatings applied by thermal spraying have been known for a long time for a plurality of applications. Thus, for example, surfaces of cylinder running surfaces lubricated by oil in vehicle motors have been coated for a very long time, amongst other things, by plasma spraying, wherein the layer significantly reduces, in particular the coefficient of friction which is effective between the piston ring and the cylinder wall, such that the wear of the piston rings and the cylinders is significantly reduced which leads to an increase of the running performance of the motor, an increase of the period of the interval between maintenance, for example, an oil change, and not least to a significant increase in the performance of a motor.
Further typical applications of surfaces applied by thermal spraying are the coating of turbine parts with wear protective layers and thermal barrier layers of components of oil or dry lubricated bearings, such as the coating of crank bearings or other work pieces which are subjected to particular physical, chemical or thermal loads. Depending on the purpose the layer has to satisfy, very particular materials, generally in the form of spray powders or spray wires are used, which materials have the required specific properties and composition in order to generate the required properties of the surface layer to be sprayed.
A different application example is the provision of a friction surface at a surface which serves precisely the reverse purpose and should increase the friction between the substrate and a friction partner to be brought into contact with the substrate, with the substrate itself simultaneously being protected against wear and/or damage.
Thus, it is, for example, already known from the DE 43 21 713 A1 to provide brake discs for vehicles having a thermal spray layer, in order to, on the one hand, increase the braking effect on braking the vehicle and to, on the other hand, simultaneously protect the substrate, this means the brake disc, itself from direct wear and in this way to finally also increase the lifetime of the brake disc.
In this connection, the requirements on the braking systems, in particular in the automotive industry, and also in the e.g. aeronautical field have continuously increased. The braking systems must also controllably and reliably brake relatively heavy vehicles or aeroplanes, moreover at high speeds and at adverse conditions, such as aquaplaning. This represents high demands in effort and cost, not only with respect to e.g. the adhesive tensile strength of the friction coating at the braking disc, but also with respect to the physical parameters, such as the micro-hardness and the macro-hardness, the thermal stability of the properties of the friction layers etc. In this connection, the friction layers alone should be manufacturable relatively simply and if possible with established methods already for economic reasons and the materials used for the coating should have a simple structure in their chemical composition and should be able to be manufactured with comparatively little demand in effort and cost. Moreover, at the same time the friction layers should simultaneously have a long lifetime, this means, wear as little as possible during operation from a mechanical point of view and besides this also be sufficiently resistant with respect to the very high temperature developments on braking and also with respect to chemical attacks, such as e.g. be as resistant as possible with respect to corrosion, such that the long periods between maintenance and/or periods between replacement can be realized.
The solutions known from the state of the art in this connection can generally only optimize one specific or at most a few of the previously mentioned aspects, wherein a compromise has to be made with respect to the other properties.
In order to at least partly balance out the disadvantages, in part comparatively demanding coating methods having complicated layer systems have been suggested with respect to the brake discs. Thus, in the DE 10 2009 008 114 A1 a brake disc having a tungsten carbide based coating is suggested, wherein the tungsten carbide coating, applied by means of thermal spray method, has to be post-treated by means of carbonizing, oxidizing, gas nitro carbonizing or a different gas or plasma-based method.
The post-treatment in this connection is compulsory necessary in order to achieve a hardness of the friction area close to the surface in that a wear resistant and corrosion resistant hardened surface layer is formed through the diffusion of the atoms from the plasma and/or gas phase.
This method is naturally very demanding in effort and cost, since after the thermal coating of the brake discs a gas treatment method has to be switched downstream in a compulsory manner which unnecessarily complicates the manufacture and makes it expensive.